1. Field of the Invention
The present invention relates generally to gray control rod assemblies for nuclear reactors and, more particularly, is concerned with an improvement to reduce swelling and heating in the neutron absorbing material in a dashpot region of a fuel assembly guide thimble when a gray control rod is fully inserted into the nuclear reactor core.
2. Description of the Related Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies, each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles, and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission, and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shut down, is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux, and thus, the heat output of the core, is normally regulated by the movement of the control rods into and from the guide thimbles.
One common arrangement utilizing control rods in association with the fuel assembly can be seen in U.S. Pat. No. 4,326,919 to Hill and assigned to the assignee of the present invention. This patent shows an array of control rods supported at their upper ends by a spider assembly, which in turn is connected to a control rod drive mechanism that vertically raises and lowers (referred to as a stepping action) the control rods into and out of the hollow guide thimbles of the fuel assembly. The typical construction of the control rod used in such an arrangement is in the form of an elongated metallic cladding tube having a neutron absorbing material disposed within the tube and with end plugs at opposite ends thereof for sealing the absorbent material within the tube. Generally, the neutron absorbing material is in the form of a stack of closely packed ceramic or metallic pellets which, in the case of a B4C absorber material, only partially fill the tube, leaving a void space or axial gap between the top of the pellets and the upper end plug which defines a plenum chamber for receiving gases generated during the control operation. A coil spring is disposed within this plenum chamber and held in a state of compression between the upper end plug and the top pellet so as to maintain the stack of pellets in their closely packed arrangement during stepping of the control rods.
Thus, control rods affect reactivity by changing direct neutron absorption. Control rods are used for fast reactivity control. A chemical shim, such as boric acid, is dissolved in the coolant to control long-term reactivity changes. More uniformly distributed throughout the core, the boron solution leads to a more uniform power distribution and fuel depletion than do control rods. The concentration of boron is normally decreased with core age to compensate for fuel depletion and fission product build up.
The build up of fission products, such as xenon-135, reduces reactivity by parasitically absorbing neutrons, thereby decreasing thermal utilization. The xenon-135 (hereinafter referred to as just “xenon”) is removed by neutron absorption, or by decay. Upon a reduction in core power (such as during load follow, which is a reduction in reactor power in response to a reduction in power demand), fewer thermal neutrons are available to remove the xenon. Therefore, the concentration of xenon in the core increases.
This increase in xenon concentration which accompanies a reduction in core reactivity is usually compensated for by either decreasing the concentration of boron dissolved in the core coolant, or by withdrawing the control rods from the core. However, both of these methods have drawbacks. Changing the boron concentration requires the processing of coolant, i.e., water, which is difficult and not desired by the utility, especially towards the end of core life. Removal of control rods means that the core's return to power capability is reduced and peaking factors are increased.
The usual solution to this problem is to have several banks of reduced reactivity worth rods, known as gray rods, in the core at full power and which are available for removal at reduced power to compensate for xenon build up. In an advanced passive nuclear plant, known as the AP1000 reactor, designed by the assignee of this invention, gray rods with a relatively low reactivity worth will be used to compensate for gross changes in core reactivity during steady state and load follow operations. This operational strategy will result in gray control rods constantly being cycled in and out of the core, both at full power steady state and reduced power transient conditions. During this operation, one or more gray rod banks may become fully inserted for extended periods of time, with the control rod tips located in the dashpot region of the guide thimble tubes. The dashpot region is a reduced inside diameter section in the lower portion of the thimble guide tubes that slows the descent of the control rods when they are dropped into the core to lessen the impact of the spider on the top nozzle of the fuel assembly. The dashpot region at the bottom of each guide thimble tube is approximately two feet (0.61 meters) long. The coolant flow rate and coolant cross-sectional area in the dashpot region are slightly lower than in the remainder of the guide thimble tube when the control rod is inserted.
The anticipated technical challenges associated with this operating strategy may include:                the potential for mechanical interference or binding between the gray control rod tip and the guide thimble tube in the dashpot region, due to radiation-induced swelling of the absorber material in some of the gray rods as a result of long-term rodded operation at power;        boiling of the coolant in the dashpot region when the gray rods are fully inserted, leading to the potential for increased guide thimble corrosion rates and decreased heat transfer from the interior of the control rod; and        fuel integrity challenges due to short-term local power changes when the gray control rods are ultimately withdrawn.        
Accordingly, it is an object of this invention to overcome the enlargement of the control rod cladding diameter due to the radiation-induced swelling of the neutron absorbing material within the control rod in the region of the control rod cladding that encounters the dashpot when the control rod is fully inserted.
It is a further object of this invention to reduce the heating of the control rod tip in the region of the dashpot when the control rod is fully inserted.
It is an additional object of this invention to reduce the rapid change in reactivity that occurs in the core when the gray control rods are slowly removed.